Silver is a noble metal that has many uses including in jewellery, decoration, coinage, cutlery, dentistry, medicine and electronics. For some of these applications, silver is too soft to be used in its pure state and it is typically alloyed with other metals such as copper to give a harder, more durable material. For example, sterling silver which is widely used in jewellery is 92.5% silver and 7.5% copper.
It is well known that under normal atmospheric conditions, silver and some silver alloys develop a dark sulphide-containing film known as tarnish on exposed surfaces. A specific problem of silver-copper alloys such as sterling silver is the development of dark staining when the alloy is annealed, brazed or soldered. These stains, which are known as firestain, are caused by oxidation of copper on and below the surface of the article which easily occurs in thermal processes. Firestain below the surface of the article cannot easily be removed by chemical or mechanical means. Firestain caused by soldering tends to be even more unsightly due to the high temperatures involved. Furthermore, if the depth of firestain exceeds more than about 0.025 mm, the alloy may be difficult to solder because it is difficult to remove the oxide by pickling and the oxidized surface will then not be properly wetted by the solder.
One solution to these problems is proposed in GB Patent No 2255348 which discloses a ternary alloy of silver in which part of the copper is replaced by germanium. It is claimed that this substantially reduces the occurrence of firestain and also reduces the development of tarnish during exposure to normal atmospheric conditions due to the formation of a thin layer of germanium oxide on exposed surfaces. A germanium content of 1.5 to 3.0% is preferred. Ternary silver alloys of this type have achieved commercial success and are sold under the trade mark ARGENTIUM®.
However, the present inventors have found that this solution does not eliminate the problem of firestain. Further, in applications where it is necessary to melt this alloy, it is found that a film forms on the surface making it difficult to estimate the temperature of the alloy visually as is conventionally done in this industry in order to control the casting process. There remains a need for a silver alloy with improved tarnish and firestain resistance.